1. Field of the Invention
This invention relates to a serial-parallel type a/d converter for converting an analog signal into a digital signal.
2. Description of the Prior Art
A serial-parallel type a/d converter converts an analog signal through upper-digit converting and subsequent lower-digit converting steps. FIG. 8 is a block diagram of such serial-parallel type a/d converter. In FIG. 8, an analog input signal 1 is applied to an upper-digit converting block 220. The converted data from the upper-digit converting block 220 is sent to an indicating logic circuit 224 and a d/a converter 221 converting the upper-digit data into another analog signal which is subtracted from the input analog signal by a subtractor 222. The subtracted signal is converted by a lower-digit converting block 223. The logic circuit 224 processes the converted data from the upper-digit and lower digit-converting blocks 220 and 223 to output final data.
Such conventional serial-parallel type a/d converters comprise less comparators than a parallel type a/d converter. However, there are drawbacks that it is necessary to respectively adjust a gain and an offset voltage of the subtractor 222 to full-scale voltage range and offset voltage of the lower-digit converting block 223 accurately; to respectively adjust full-scale voltage range and offset voltage of the d/a converter 221 to those of the upper-digit converting block 220 accurately. Thus, there are many adjusting points so that converting accuracy is unstable among manufactured a/d converters and it is difficult to manufacture this converter as a monolithic integrated circuit. There are also drawbacks that high-speed converting in this conventional a/d converter is difficult because a settling time of the subtractor 222 of an operational amplifier shows long settling time.